38 PROCEEDINGS OF THE NATIONAL MUSEUM VOL, 125 
radiation that produced these species, a form similar to P. mexicanus 
gave rise to P. hobbsi, which evolved a more advanced type of re- 
productive system and stands at the second evolutionary level. 
Pterodrilus cedrus is the survivor of a stock with dorsal ridges and 
projections that gave rise to the other main lineage composed of 
P. distichus, P. alcicornus, and, at a more primitive stage of the 
development of the reproductive systems, P. stmondsi, the members 
of the second radiation. 
PLACES OF ORIGIN AND MIGRATIONS.—When the distribution of the 
species of Pterodrilus (figs. 9, 10) is considered along with the hy- 
pothesis of their phylogeny that has been sketched here, some con- 
clusions immediately emerge. The phylogenetically primitive species 
are scarce and localized. The most primitive of all, P. missouriensis, 
is known from a single location in the headwaters of the Gasconade 
River in Missouri. The more abundant but still relatively scarce 
P. mezicanus is essentially confined to the White River system in 
Missouri and Arkansas since it is otherwise known only from the 
nearby St. Francis River in Missouri, a tributary to the Arkansas 
River in Oklahoma, and Veracruz, Mexico. Pterodrilus choritonamus 
and P. cedrus are inhabitants of tributaries to the Cumberland River 
in the Eastern Highland Rim and Nashville Basin regions of Ten- 
nessee, P. hobbsi is a widespread and successful species of the Cumber- 
land and Tennessee River systems with outliers in the Big Sandy 
and New Rivers. Of the species of the lineage with dorsal projections 
on multiple segments, the most primitive, P. simondsi, is localized 
in the Hiwassee River drainage of the Tennessee basin; P. distichus is 
a species of the Kentucky River that has crossed the Ohio to invade the 
eastern Great Lakes and St. Lawrence drainages; P. alcicornus is 
found in the Tennessee and New River systems, again with outliers 
to the east and north in the Savannah, Roanoke, James, and Big 
Sandy Rivers. 
The ancestral home of the genus Pterodrilus most likely is in the 
headwaters of the Cumberland River in the Eastern Highland Rim 
region of Tennessee. Two of the four most primitive species, P. 
choritonamus and P. cedrus, still persist as phylogenetic and geographic 
relicts in this region. The other species are arranged radially around 
this center in a fashion that almost requires that their ancestors come 
from the Cumberland (fig. 12). 
The same general region was the postulated home of the ancestors 
of the host animals, primitive Procambarus crayfishes that gave rise 
to the genera Orconectes and Cambarus, with Orconectes spreading 
mostly to the north and west, Cambarus to the east and south, and 
some stocks of Procambarus southwestward into Mexico (Hobbs, 1967, 
p. 15). The modern host relationships of species of Pterodrilus can 
